1. Field of the Invention
The invention relates to information storage media, in particular storage media useful with holographic systems.
2. Discussion of the Related Art
Developers of information storage devices and methods continue to seek increased storage capacity. As part of this development, so-called page-wise memory systems, in particular holographic systems, have been suggested as alternatives to conventional memory devices. Page-wise systems involve the storage and readout of a representation, e.g., a page, of data. Typically, recording light passes through a two-dimensional array of dark and transparent areas representing data, and the holographic system stores, in three dimensions, holographic representations of the pages as patterns of varying refractive index in a storage medium. Holographic systems are discussed generally in D. Psaltis et al., xe2x80x9cHolographic Memories,xe2x80x9d Scientific American, November 1995, the disclosure of which is hereby incorporated by reference. One method of holographic storage is phase correlation multiplex holography, which is described in U.S. Pat. No. 5,719,691 issued Feb. 17, 1998, the disclosure of which is hereby incorporated by reference.
FIG. 1 illustrates the basic components of a holographic system 10. System 10 contains a modulating device 12, a photorecording medium 14, and a sensor 16. Modulating device 12 is any device capable of optically representing data in two-dimensions. Device 12 is typically a spatial light modulator that is attached to an encoding unit which encodes data onto the modulator. Based on the encoding, device 12 selectively passes or blocks portions of a signal beam 20 passing through device 12. In this manner, beam 20 is encoded with a data image. The image is stored by interfering the encoded signal beam 20 with a reference beam 22 at a location on or within photorecording medium 14. The interference creates an interference pattern (or hologram) that is captured within medium 14 as a pattern of, for example, varying refractive index. It is possible for more than one holographic image to be stored at a single location and/or for holograms to be stored in overlapping positions, by, for example, varying the angle, the wavelength, or the phase of the reference beam 22, depending on the particular reference beam employed. Signal beam 20 typically passes through lens 30 before being intersected with reference beam 22 in the medium 14. It is possible for reference beam 22 to pass through lens 32 before this intersection. Once data is stored in medium 14, it is possible to retrieve the data by intersecting reference beam 22 with medium 14 at the same location and at the same angle, wavelength, or phase (depending on the multiplexing scheme used) at which reference beam 22 was directed during storage of the data. The reconstructed data passes through lens 34 and is detected by sensor 16. Sensor 16 is, for example, a charged coupled device or an active pixel sensor. Sensor 16 typically is attached to a unit that decodes the data.
The capabilities of such holographic storage systems are limited in part by the storage media. Iron-doped lithium niobate has been used as a storage medium for research purposes for many years. However, lithium niobate is expensive, is poor in sensitivity (1 J/cm2), has relatively low index contrast (xcex94n of about 10xe2x88x924), and exhibits destructive read-out (i.e., images are destroyed upon reading). Alternatives have therefore been sought, particularly in the area of photosensitive polymer films. See, e.g., Selected Papers on Holographic Recording, H. J. Bjelkagen, ed., SPIE Press, Vol. MS 130 (1996). The materials described in this set of articles generally contain a photoimageable system containing a liquid monomer material (the photoactive monomer) and a photoinitiator (which promotes the polymerization of the monomer upon exposure to light), where the photoimageable material system is located within an organic polymer host matrix that is substantially inert to the exposure light. During writing of information into the material by exposure to radiation in selected areas, the monomer polymerizes in the exposed regions. Due to the lowering of the monomer concentration caused by induced polymerization, monomer from the dark, unexposed regions of the material diffuses to the exposed regions. The polymerization and resulting concentration gradient create a refractive index change, forming the hologram representing the data. Typically, the system is then fixed by a flood cure exposure, which destroys any remaining photosensitivity in the medium. (For further discussion of the recording mechanism, see xe2x80x9cOrganic Photochemical Refractive Index Image Recording Systemsxe2x80x9d in Advances in Photochemistry, Vol. 12, John Wiley and Sons (1980).) Most holographic systems of this type are based on photopolymerization of free-radical photoactive monomers such as acrylate esters. See, for example, U.S. patent application Ser. No. 08/698,142 (our reference Colvin-Harris-Katz-Schilling 1-2-16-10), the disclosure of which is hereby incorporated by reference.
While such photopolymer systems provide useful results, they exhibit changes in dimension due to shrinkage induced by polymerization of the photactive monomers. Dimensional changes are also caused by thermal expansion. (Typical linear coefficient of thermal expansion values for these systems range from about 100 to about 300 ppm/xc2x0 C.) These dimensional changes, while small, tend to distort the recorded holographic gratings, degrade the fidelity with which data is capable of being recovered, and thereby limit the density of data which the polymer is able to support. Some attempts to overcome these dimensional changes have led to experimentation with porous glass matrices containing a photoimageable system. See, e.g., U.S. Pat. Nos. 4,842,968 and 4,187,111; V. I. Sukhanov et al., xe2x80x9cSol-Gel Porous Glass as Holographic Medium,xe2x80x9d Journal of Sol-Gel Science and Technology, Vol. 8, 1111 (1997); S. A. Kuchinskii, xe2x80x9cPrinciples of hologram formation in capillary composites,xe2x80x9d Opt. Spectrosc., Vol. 72, No. 3, 383 (1992); S. A. Kuchinskii, xe2x80x9cThe Principles of Hologram Formation in Capillary Composites,xe2x80x9d Laser Physics, Vol. 3, No. 6, 1114 (1993); V. I. Sukhanov, xe2x80x9cHeterogeneous recording media,xe2x80x9d Three-Dimensional Holography: Science, Culture, Education, SPIE Vol. 1238, 226 (1989); V. I. Sukhanov, xe2x80x9cPorous glass as a storage medium,xe2x80x9d Optica Applicata, Vol. XXIV, No. 1-2, 13 (1994); and J. E. Ludman et al., xe2x80x9cVery thick holographic nonspatial filtering of laser beams,xe2x80x9d Opt. Eng., Vol. 36, No. 6, 1700 (1997).
U.S. Pat. No. 4,842,968, for example, discloses a process in which a porous glass matrix is immersed in a photoimageable system, such that the photoimageable system diffuses into the open pores of the matrix. After exposure to light, the unexposed, i.e., non-polymerized, portions of the photoimageable system must be removed from the pores with a solvent. Typically, a different material offering desirable refractive index contrast is then introduced into the emptied pores. It is only after these steps that a readable hologram is formed. (While the initial irradiation step tended to form a latent image in these previous matrix-based media, the latent image could not be read non-destructively by the same wavelength of light used for recordation, i.e., the reference beam could not be used for readout. Thus, no hologram was considered to have been formed. As used herein, the term readable hologram indicates a pattern capable of being non-destructively read by the same wavelength of light used for recordation.)
While glass matrices offer desirable rigidity and structural integrity, as well as formation of relatively thick, e.g., greater than 1 mm, media, the ""968 patent illustrates several practical drawbacks encountered in such matrix-based recording media. Specifically, complex chemical treatments with solvents are required after exposure to remove reacted or unreacted material in order to attain a readable hologram. These treatments are undesirable from a commercial usability standpoint, and also tend to cause unwanted non-uniformity in the material.
Thus, while progress has been made in fabricating photorecording media suitable for holography, further progress is needed. In particular, media that exhibit improved chemical and structural integrity, yet which are capable of being formed by relatively simple processes, are desired.
The photorecording medium of the invention offers desirable structural stability, and is capable of being formed by processes less complicated than previous media. Specifically, the medium is fabricated by a relatively straightforward process of mixing the photoimageable system and a polymeric matrix precursor, and curing the precursor in situ. (In situ indicates that the matrix is cured in the presence of the photoimageable system. The matrix is considered to be formed when the photorecording material (i.e., the matrix, photoimageable system, and any other additives) exhibits an elastic modulus of at least about 106 Pa. Curing indicates reacting the matrix precursor such that the matrix provides at least this elastic modulus.) The matrix and photoimageable system are selected to exhibit particular properties relative to each other. First, the photoactive monomer of the photoimageable system and the matrix precursor polymerize by independent reactions, such that the curing step does not interfere with or inhibit hologram formation. Second, the matrix polymer and the photoimageable system are selected such that (a) the matrix precursor and photoimageable system are substantially soluble, i.e. miscible, but (b) during the cure, as the matrix precursor polymerizes, the resulting polymer and the photoimageable system phase separate. (Phase separated indicates that there exist distinct regions predominantly containing the photoimageable system, where at least 75 vol. % of the originally added photoimageable system is found in such regions, and interfacial regions between these distinct regions and the matrix polymer containing varying amounts of matrix material and photoimageable system.) Yet, in spite of this phase separation, the matrix/photoimageable system exhibits low light scattering, such that holographic writing and reading is possible.
(Independent reactions indicate: (a) the reactions proceed by different types of reaction intermediates, e.g., ionic vs. free radical, (b) neither the intermediate nor the conditions by which the matrix is polymerized will induce substantial polymerization of the photoactive monomer (substantial polymerization indicates polymerization of more than 20% of the monomer), and (c) neither the intermediate nor the conditions by which the matrix is polymerized will induce a non-polymerization reaction of the monomer that either causes cross-reaction between monomer and the matrix or inhibits later polymerization of the monomer. The photoinitiator also substantially survives the matrix formation process. Low light scattering indicates that the Rayleigh ratio in 90xc2x0 light scattering of a wavelength used for hologram formation (R90xc2x0) is less than about 7xc3x9710xe2x88x923. The Rayleigh ratio (Rxcex8) is a conventionally known property, and is defined as the energy scattered by a unit volume in the direction xcex8, per steradian, when a medium is illuminated with a unit intensity of unpolarized light, as discussed in M. Kerker, The Scattering of Light and Other Electromagnetic Radiation, Academic Press, 1969, 38. The Rayleigh ratio is typically obtained by comparison to the energy scattered by a reference material having a known Rayleigh ratio.)
Thus, unlike previous polymer media, which tend to contain a substantially homogeneous dispersion of matrix polymer and photoimageable system, the medium of the invention contains a matrix and photoimageable system which are phase separated, yet still exhibit low light scattering. Selection to obtain this phase separation is based on relative properties of the matrix and photoimageable system, e.g., solubility and kinetics, as discussed, for example, in the art of aerogels and polymer-dispersed liquid crystals. In addition, parameters such as refractive index contrast and the wavelength being used affect the Rayleigh ratio. The medium is useful for holography, and, advantageously, the matrix is rigid enough to reduce the extent of bulk shrinkage due to image formation, as exhibited by previous polymer-based media. A strength of at least 106 Pa, as measured by shear rheology, is particularly desirable, this measure used as a crude predictor of deleterious optical change upon writing. It has been found that a cross-linked melamine-formaldehyde resin matrix with a photoactive monomer of N,N-dimethylacrylamide offers useful rigidity, phase separation, and holographic properties.